Sheathed heaters and cartridge heaters have been conventionally used. These heaters include a heating element in the form of a metal wire, a cover for sealing the heating element, and an electrical insulating material constituted by an oxide, e.g., magnesium oxide, that is filled between the heating element and the cover. The cover is composed of a metal portion and an electrical insulator portion through which lead wires of the heating element extend. Joule heat is generated at the heating element by energizing the heating element via the lead wires.
Because nickel, chromium, and iron are normally used as part of the composition of the heating element and the metal portion of the cover, the heating element and the metal portion of the cover will oxidize when the heating element and the metal portion of the cover are used for a long time at temperatures as high as 850° C. or more. Because oxygen in the air disposed within the sealed heater and oxygen in the electrically insulating material is reduced as a result of the oxidization, the pressure within the sealed cover is reduced. Evaporation and dispersal of nickel, chromium, and iron components of the heating element and the metal portion of the cover is accelerated as a result of the pressure reduction.
In this event, chromium oxide forms on the surface of the heating element and the metal portion of the cover, which is made of nickel, chromium, and iron, as a result of use at high temperature, and the chromium oxide, chromium, nickel, and iron will evaporate and disperse into the electrically insulating material.
Thus, a reduction reaction of the electrically insulating material and a phenomenon known as blackening of the electrically insulating material will result, because of the dispersal of conductive chromium, chromium oxide, nickel, and iron, which evaporate and are released from the heating element and the metal portion of the cover in the electrically insulating material, thereby accelerating the deterioration of the insulation resistance of the electrically insulating material.
As a result, an extraordinarily large leakage current sometimes flows from the heating element to some locations of the metal portion of the cover via the electrically insulating material when the sheathed heater or cartridge heater is being used, and a large amount of Joule heat is locally generated at the heating element, thereby developing an extraordinarily high local temperature, which cause problems of breakage occuring in the heating element that has become thin due to release of the chromium, nickel, and iron components and the cover may melt or break.
The present invention has been conceived taking such points into consideration, and it is an object of the invention to make it possible to use heaters, such as sheathed heaters and cartridge heaters, at temperatures higher than the prior art, by suppressing deterioration of the insulation resistance of an electrically insulating material that is filled between a heating element and a cover.